The early satellite communication system designs employed an area coverage beam which provided interconnections on either a time-division multiple access (TDMA) basis or a frequency-division multiple access (FDMA) basis. Such designs had the disadvantage of low antenna gain and frequency reuse only by the use of polarization techniques. More recent designs use (a) multiple narrow-angle fixed spot beams with onboard satellite switching to provide frequency reuse, high capacity, and high antenna gain, (b) a single scanning beam to provide high antenna gain, (c) the combination of an area coverage beam and multiple narrow-angle fixed spot beams to provide high capacity, and (d) the combination of multiple narrow-angle fixed spot beams and a single scanning beam with on-board satellite switching.
A typical prior art design is shown in U.S. Pat. No. 3,711,855, issued to W. G. Schmidt et al on Jan. 16, 1973, which illustrates a conventional multiple-transponder satellite with n transponders for n or more ground stations where each transponder covers a particular portion of the frequency spectrum and no two ground stations may concurrently transmit in the same frequency band. Another design is shown in U.S. Pat. No. 3,924,804, issued to W. G. Schmidt et al on Dec. 23, 1975, where a plurality of receive spotbeam antennas are selectively connected to a plurality of transmit spotbeam antennas by an on-board switching matrix. Additionally, several other separate receive and transmit spotbeam antennas are connected to a common receiver and transmitter, respectively, by a respective on-board input and output switch.
An article "Analysis of a Switch Matrix for an SS/TDMA System" by Y. Ito et al in Proceedings of the IEEE, Vol. 65, No. 3, March 1977 at pp. 411-419 discloses a technique which provides a most efficient utilization of a frame period with n.sup.2 -n numbers of switchings at most, where n is the number of beams in the SS/TDMA system.
A major problem in multibeam satellite design is one of transponder reliability. Unlike area coverage systems wherein the allocated band is divided among several transponders and service is provided via frequency division multiple access, it is desirable to serve each spot beam of a multibeam satellite system with a single transponder. With this approach, the required number of transponders is kept from becoming prohibitive, and the weight of the communications subsystem is minimized. However, sufficient redundancy must be provided to ensure high reliability for each transponder since single failures would preclude continuing service to the area serviced by that transponder. By contrast, for area coverage systems using frequency division multiple access, isolated failures merely cause a slight increase in the demand presented to the surviving transponders.
A second problem in multibeam satellite systems concerns efficient utilization of the satellite transponders. In general, the traffic demands from the various coverage areas, or footprints, are nonuniform. Thus, to utilize each transponder fully, the capacity of each must be tailored to the traffic demand of the area covered by that transponder. A technique for achieving such a custom fit has been disclosed in the article "An Efficient Digital Satellite Technique for Serving Users of Differing Capacities" by H. W. Arnold in ICC Conference Record, June 12-15, 1977, Chicago, Ill., Vol. 1, at pp. 6.1-116 to 6.1-120 wherein the bit-rate of each beam is selected as a fixed multiple of some basic rate. At the satellite, each up-link beam is demultiplexed into several basic rate bit streams, switched, and then remultiplexed into down-link beams. One disadvantage of this scheme is that on-board demodulation and remodulation is required. However, a more serious disadvantage in such a system is the need for nonidentical transponders which precludes sharing of a common pool of spare transponders among all beams, and the reliability of the system suffers.
A third problem of multibeam satellites involves means of accessing traffic from areas not within the footprint of some spot beam. Several solutions have been proposed in the article "Spectral Reuse in 12 GHz Satellite Communication Systems" by D. O. Reudink et al in ICC Conference Record, June 12-15, 1977, Vol. 3 at pp. 37.5-32 to 37.5-35 involving sharing the spectrum between spot beams and an area coverage beam. These have the disadvantage that the area coverage transponders are different from the spot beam transponders and have higher power requirements to compensate for the loss of antenna gain. Also, the fixed spot beam transponders, when assumed identical, are not matched to traffic requirements of the area served.
Another solution to the access problem as disclosed in the article "A Scanning Spot Beam Satellite System" by D. O. Reudink et al in Bell System Technical Journal, Vol. 56, No. 8, October 1977 at pp. 1549-1560 involves the use of a steerable spot beam which can be rapidly scanned across the entire service region via a phased array antenna, thereby providing universal coverage. When used in conjunction with a multitude of fixed spot beams, the resulting hybrid system has the advantages of frequency reuse, high antenna gain, and identical transponders. However, such a hybrid system does not utilize the transponders efficiently because of nonuniform traffic demands from the various ground areas covered.
The problem, therefore, remaining in the prior art is to provide a satellite system concept whereby the resources such as, for example, the available power and transponders at the satellite are most efficiently matched to the instantaneous terrestrial traffic patterns, while providing uniform coverage over a wide service area, with identical or nearly identical transponders.